Hot rolling of metal powder



May 3, 1960 F, J, TRQTTER ETAL 2,935,402

HOT ROLLING OF METAL POWDER Filed April 15, 1954 INVENTORS GerhardNaeser' Frederick J. Trotter Richard B. Montgomery Lincoln T. Work BY/ATTORNEYS p of the equipment.

2,935,402 nor ROLLING or METAL POWDER Frederick J. Trotter, London,England, Richard B. Montgomery, Bethel, C onn., Lincoln T. Work,Maplewood,

N.J., and Gerhard Naeser, Duisburg-Huckingen, Germany, assignors,bymesne assignments, of one-half to Mannesmann Aktiengesellschaft,Dusseldorf, Germany, a German company, and one-half to ChernetalsCorporation, New York, N.Y., a corporation of Delaware Application April15, 1954, Serial No. 423,436

3 Claims. (Cl. 75-221) Thisinvention relates to a process for convertingmetal powders into wrought metal shapes such as strips, rods, wires,sheets, and the like. More particularly, the invention relates to aprocess which comprises sintering metal powder to form a sintered barand hot rolling the sintered bar.

It is well known that metal may be recovered in powder form from scrapor from ores or from concentrates containing them in the form ofmetal'oxides or sulfides by hydro-metallurgical processes. Forexample,metal powders such as copper, nickel and cobalt can berecovere'd'i'n substantially pure form by dissolving the metal from theraw material in an aqueous solution and then heating the solution to anelevated temperature and pres- .sure with a reducing gas. The metal isprecipitated in substantially pure form, more or less pulverulentinchar- "actor. In its precipitated form, the metal'has at best ratherlimited fields of use. Generally it mustbe converted into a wroughtshape in order to be useful. Such conversion, however, is not a simplematter. Melting the pulverulent precipitate and casting into shapes'for'rolling'isnot economically desirable because such 'procedure entailslarge losses of metal due to oxidation and slaggin'g. Melting in avacuum or in a non-oxidizing or reducing atmosphere greatly increasesthe cost of oper- 'ation. Hot' extrusion of the pulverulent metal in arefducing'atm'osphere has been proposed, but this procedure also iscostly, on account of the excessivewear and tear Fabrication of themetal into wrought or other shapes by conventional techniques ofpowder:metallurgyhas not proved satisfactory, partly because such-techniquesare suitable only for making rather'small -articlesg-and partly becausethe pulverulent metal preeipitate-h'as 1110!. proved itself tobeuniformly suitable for fabrication by the methods of powder metallurgy.:It has also'been proposed to roll the metal powder directly betweencold pressure rolls to form a raw product and then subsequently sinterthe same and further roll the sintered product to produce wrought metalshapes, .but this technique requires synchronization of 'the formingrolls and the sintering operation, and, for a given vsintering period;long and costly'furn'aces are called, for. When-usingthis-techniquet-he'metal powder must have *its moisture and, oxidecontent completely removed before it can be formed into strip or sheetform. The feed of the dry powder into the rolls must also be 'even .inorder'to prevent thin spots, and the very powerful pressure rollsthemselves must also have a minimum diameter of about 50 to 1,000 timesthe thickness of v the metal'product being rolled in order tosufliciently consolidate the metal powder.

We have discovered that despite the non-uniform bevhavi'or of thepulverulent precipitates of conventional powder metallurgy fabricationprocedures, it is possible to'secure good and consistent results in theconsolida- 'tion' of such precipitates into a bar which can be workedfurther into a wrought metal shape, and also obviate many disadvantagesof the conventional consolidation p tes Patent vention. .r noi'sture.advantageous, as a complete drying step takes upcone Patented .May 3,.1960

procedures, both from a cost standpoint and from .the standpoint ofproducing high quality and uniform'wroug'ht metal products.

Generally, according to the present invention, the metal powder is firstfed onto a suitable support in the dimensions desired, and then directlypassed to a furnace in which a controlled reducing atmosphere ismaintained, to effect at least a partial sintering of the powder to forma fairly self-sustaining sintered bar or slab. The I According to thepresent invention, thisywrought metal powder, after it has just beenprecipitated," may' first be suitably washed and passedthrough a dryer.Any conventional type dryer is suitable. The powder discharged from thedryer may contain a small percentage of mois- ,1 two.

We have found that a moisture content of from about 0.5 percent to about1.0 percent does not interfere with the subsequent consolidation of thepowder. The moisture-containing powder may be directly fed from thedryer onto a suitable supportand directly passed into a furnace to,effect sintering thereofland otherwise consolidated according to theprocess of the present in- The sintering operation readily eliminatesThis feature of the present invention is highly siderably. more time andis costly. Furthermore, when completely drying the metal powder, thedanger of oxidation, and of incipientaggregation at the comparativelyhigh temperatures required, both of which interfere with subsequentoperations, is always present, and in order to avoid this it is oftennecessary to dry in a reducing atmosphere. It is thus possible tomaintain the temperature of the dryer sufliciently low so as to preventany oxidation of the particular metal powder being dried andconsolidated. Using such a temperature eliminates any necessity forusing a protective gas atmosphere to prevent oxidation, and such aprocess makes it possible to utilize such a drying procedure andmaintain the process on a sound and commercial economical basis, whileproducing good and uniform wrought metal products The temperature usedwhich will elfect a sufficient drying of the metal powder and yetprevent oxidation without the use of a protective atmosphere, will, ofcourse, vary depending upon the particular product being dried andconsolidated. Such temperature will be readily apparent to those skilledin the art. When drying and consolidating copper powder, for example,the temperature of the dryer should not exceed about 110 C. At thistemperature the copper powder may be efliciently dried for the purposesof this invention without adversely affecting the powder.

After the powder has been sufiiciently washed and dried, it is spreadupon a suitable support, such as a continuous metal belt, in the desireddimensions. The powder emerging from the dryer is advantageously fedonto the continuous belt while it is still hot to save heat and preventoxidation thereof. The thickness and width of the metal powder mass willbe determined in part by the particle size of the powder beingconsolidated, and the'dimensions of the powder mass will also determineto a certain extent the temperature used to efiect the heatingof thepowder and the time necessary to sufficiently heat the same. I It isobvious that since the metal powder mass is very porous, it. isa poorheat conductor; and the larger the powder mass is in thickness andwidth, the longerit will take to completely and sufficiently heat thepowder mass throughout its cross-sectional area. a We have found that apowder mass of about 2 inches in thickness and about 2 inches 'inwidthmay be satisfactorily heated, for any particular metal, at fairly lowtemperatures in two to three minutes. Smaller powder masses may ofcourse be heated in less time. Due to the fact that when the metalpowder is first deposited on a continuously-moving belt, it is at timesunevenly distributed thereover, it is also advantageous to smooth themetal powder mass by some suitable means such as low-pressure rolls, oreven a doctor blade.

Upon heating the meal powder mass, any remaining moisture is firstreadily driven from the still porous mass. To prevent oxidation theheating should take place in a reducing atmosphere. Hydrogen gas is anexample of a suitable reducing atmosphere and may be supplied to afurnace by any suitable and conventional means. The heating of thepowder mass causes the powder particles to cohere and form a fairlyrigid and strong coherent porous bar. This bar may be further heated toa higher temperature, at or about the sintering temperature of theparticular metal being consolidated, and directly fed while still hotbetween a pair of powerful pressure rolls to consolidate the same toalmost complete densification. This powerful rolling action, however,should be so regulated and applied as to prevent any substantialelongation of the bar being compressed. If the pressure of this rollingaction is sufliciently high to cause substantial elongation of the bar,cracking often occurs in the bar rendering it useless for many purposes.After the powerful rolling action, the bar may again be heated to atemperature at or above the sintering temperature of the metal beingconsolidated, and again compressed between powerful pressure rolls tocompletely densify the bar. In this latter compressing operation, thepressure may be, and advantageously is, sutficient to cause substantialelongation of the bar. During this additional rolling step there is noserious tendency for the bar to crack when applying sufficient pressureto cause elongation thereof.

When the loose powder mass is first heated and the particles cohere, themetal powder shrinks to a considerable extent throughout its thickness,but the shrinkage in a lateral direction, or along the width of thepowder mass, is very slight. For example, we have found that a powdermass of about two inches in thickness and about two inches in widthshrinks when heated to about an inch and a quarter in thickness and theshrinkage along the width (and length) is negligible. We have also foundthat due to the shrinkage of the loose metal powder, the resultingsintered bar may contain irregularities, and when further heated anddirectly fed between a pair of powerful pressure rolls, thin spots orareas might develop in the resulting bar which are difficult, if notimpossible, to remove. We have found that if the bar, after it has beenheated sufficiently to cause the metal particles to cohere and before itis heated to a temperature above the sintering temperature, is firstslightly or partially compressed or consolidated to a modest extentsulficiently to remove the irregularities or unevenness, that the barmay be further heated and completely consolidated into a uniform densitywithout the presence of thin areas.

Another advantage of this slight consolidation resides in the fact thatthe even bar, partially consolidated, is more rapidly heated throughoutto higher temperatures due to the elimination of some of the porosity ofthe bar. In all cases where the hot bar is to be rolled, it isadvantageous to heat the rolls to prevent chilling of the hot bar. Wehave found that if the rolls are heated to a temperature of about 400C., that is sufiicient to prevent chilling of the hot bar for thepurposes of this invention.

For a more detailed description of the invention, reference may be hadto the drawing in which:

Figure 1 schematically depicts an apparatus for carrying out the processof the present invention, and

Figure 2 schematically depicts an alternative apparatus for practicingthe process of the present invention.

Although any metal powder may be processed into a wrought metal shape,copper powder is used as illustrative in the following detaileddescription, in order to more particularly point out the details of theprocess and set forth a specific illustrative procedure.

Referring to Figure 1 in the drawing, the wet copper powder, after ithas been precipitated, is placed in a centrifuge 1 to wash the powderfree of any contaminants. The powder, when taken from the centrifugeusually contains about 5%-10% moisture. The moist powder is then passedto a suitable drying apparatus such as the rotary dryer 2 shown in thedrawing. The copper powder emerging from the dryer contains about 0.5%to 1.0% moisture. The copper powder 3 is then transferred,advantageously while still hot, from the dryer onto a suitable conveyingapparatus such as the continuous conveyor belt 4. The conveyor belt maybe composed of any suitable material such as steel, and it is onlynecessary that the material of which the conveyor is composed be able towithstand the temperature to which the metal powder is to be subjected.For example, when consolidating copper powder, it may be subjected totemperatures of about 1050 C. and the conveyor belt must be able towithstand such temperatures. The belt may either be flat or fitted withskirts extending upward to aid in retaining the powder. The advisabilityof using skirts on the belt will of course depend upon the thickness ofcopper powder desired deposited on the belt. The belt may be driven byany suitable means. In the apparatus as shown in Figure 1, the belt isdriven and guided by the rollers 5 and 6.

As it is at times diflicult to deposit the powder onto the conveyor withthe uniformity desired for subsequent operations especially when stripof considerable width is to be produced, the powder may be smoothed overor flattened by suitable means such as the rolls 7.

It has also been found that if the powder mass is subjected to tensileforces prior to the sintering and rolling thereof, that it has atendency to crack. Any cracking in the powder mass may subsequently showup in the completely consolidated product as thin or weak spots. It isthus highly advantageous to prevent any cracking of the powder massprior to the subsequent sintering and rolling operations. We have foundthat by utilizing the rolls 7, or other suitable means, to subject thepowder mass to a very mild compressive force, any tendency for thepowder mass to crack is greatly minimized. Thus the rolls 7 not onlyeven out any irregularities in the thickness of the powder being fed,but also serve to prevent cracking by making sure that the metal isproperly distributed when fed through the apparatus.

The rolls may be composed of any suitable material; however,rubber-faced rolls are preferable. After the rolls 7 have performedtheir smoothing operation on the powder, the powder is continuously fedinto a suitable furnace 8. The copper powder is subjected to atemperature of about 300-600 C. in the furnace 8. A temperature of about600 C. is preferable, and at this temperature the heating period neednot exceed 2 to 3 minutes. This heating of the copper powder causes theparticles to cohere or sinter together into a bar and considerableshrinkage of the copper powder occurs. For example, copper powdermeasuring about two inches square deposited on the belt 4 will shrink inthickness to about one and one-quarter inches while shrinkage in alateral direction will be very slight. The apparent density of the stripafter it leaves furnace 8 has been found to be about 1.5. This shrinkageof the metal powder may cause some irregularities to form in the bar, inwhich case it is advantageous at this point to remove the unevenness inthe bar by some suitable means such as by the rolls 9 as shown in thedrawings. The rolls 9 are preferably made of cast iron or steel, but anyother suitable materials may be used. These rolls merely serve thepurpose of consolidating to, a modest extent "(suflicient to remove theunevenness) the still hot porous bar, and are preferably placed at apoint in the heat treatment of the powder where the temperature thereofhas not risen above about 700 C.'

The partly compacted porous bar is then passed continuously through asecond furnace in which the bar is rapidly heated to a temperature ofabout 700 C. to about 1050 C. Due to the partial consolidation by therolls 9,,this second heat treatment may be very rapidly accomplished dueto the higherheat conductivity of the slightly compacted bar issuingfrom the rolls 9.

It is not necessary to sinter the bar; once its mass has reached therequired temperature, the bar can be passed immediatelythrough the rolls11. The rolls 11 are preferably made of cast iron and may be heated toavoid -*chilling of the bar. The bar compacted in this manner has beenfound to attain a high degree of densification j du'ring a singlepass'through the pressure rolls. In fact, a porous jc'opper bar having adensity of about 1.5 after sintering in furnace 8 has been by thepresent process compacted to an apparent density of about 8.35. Thedegree of compaction by the rolls his so adjusted that no substantialelongation of the bar takes place.

' After thebfar leaves the rolls 11, it is continuously passed through acooling chamber 12 and may then be further worked if desired byconventional rolling procedures.

Further densification and elongation may be effected by utilization ofan apparatus as shown in Figure 2. In this figure, the pressure rolls 13are identical to the pressure rolls ll'shown in Figure 1 and receive thehot porous bar from the furnace 10 as shown in Figure l to effect thefirst consolidation of the bar. The compacted bar is then the barreduces the same to finaldensity and the degree of compaction by therolls 15 is so regulated as'to effect substantial elongation andcomplete densification. The bar after it leaves the pressure rolls 15 isthen continuously passed through a cooling chamber 16 to cool the'co'pper bar. The resulting copper bar may then be further worked byconventional rolling practice as desired.

We have also found that it is advantageous to coat the wet powder as itemerges from the processing plant with a small quantity of a materialwhich will, when subjected to heat in the sintering furnace, decomposeand yield a protective reducing gaseous atmosphere. We have found that avolatile liquid hydrocarbon such as transformer oil is quite suitablefor this purpose and good results may be accomplished by incorporatingabout 0.5% to 1% of the material with the metal powder. This, of course,at least partly eliminates the necessity of supplying a protectiveatmosphere from an external source and eliminates the necessity of extraequipment and specially designed furnaces.

The furnaces may be heated in any manner desired, but it is preferableto heat the low temperature furnaces (up to 600 C.) by gas and thehigher temperature furnaces by electricity for economic reasons.

The furnaces 8 and 10 as shown in Figure 1 are separate; however, theymay be joined together or be interconnected so as to make one singlelong furnace'and the rolls 9' may be placed inside the furnace. In thisembodiment the furnace is gradually heated from a temperature of between300 C. and 600 C. in the first part of the furnace and the latter endkept at a temperature of about 700 C. to about 1050 C. in which case therolls 9 should be placed at a position in the furnace at which thetemperature is about below the sin- ,tering temperature of the metal,which the. case of copper is about 700 C. It is also advantageous toenclosethe sintering and rolling operations by a jacket or any othersuitable means such as the jacket 17 shown in Fig. 1, in order t'ornairi-i tain the sintering and rolling steps, under an inert atm'osphere. As a practical matter, however, itlisadvantageous to enclose theapparatus by nothing more 'than suitable hoods extending from theoutlets of each'piece of treating.

apparatus to the inlet of the next. 7 For example, ai hood extendingbetween the furnaces 8 and 10 .and ove'rfthe As hereinbefore pointedout, any metal powder such as copper, cobalt, nickel, iron, lead and thelike may be converted into wrought metal shapes by sintering and rollingaccording to-the present invention. Wrought alloy products may also beproduced by employing. mixtures of metal powders. The temperatures androlling pressures to be utilized for the conversion of any particularmetal powder into a wrought metal shape will, in view i of the abovedescription and common knowledgepossessed by one skilled in the art, bereadily apparent. ample, nickel and iron powder may be processed according to the present invention by subjecting them to a temperature ofabout 800 C.-l000 C. and then subsequently raising this temperature toabout 1150 C. before effecting the densification thereof.

The present process provides a continuous'economic method for convertingmetal powders into wrought metal shapes of good and consistent quality.By the process of the present invention it is possible to continuouslyand completely process metal powder from the moment of its productioninto a consolidated wrought metal product without once cooling the metalpowder.

The resulting wrought metal product producedaccording to the presentinvention also possesses highly advantageous properties. It may be coldrolled by conventional rolling procedures and to any desired thicknesswithout annealing and generally possesses the properties of completelyannealed metal. I

We claim:

l. The process of producing fabricated wrought metal products from metalpowder which comprises heating the metal powder to form a shapedcoherent sinteredporous bar, compressing the bar while it is still .hotbetween a" pair of pressure rolls sufficiently to cause nearly complete2. The process of producing fabricated wrought metal' products from ametal powder obtained in a hot, wet state by a hydro-metallurgicalprocess which comprises drying the hot metal powder to remove the majoramount of moisture contained therein and at a temperature which isinsufficient to cause oxidation of the metal powder, distributing thestill hot metal powder after it has been dried on a continuously movingsupport in the desired dimensions to form a smooth powder mass,continuously moving the powder mass through a heating zoneto heat thepowder mass and cause coherence of themetal powder particles to form ashaped coherent sintered porous bar,

slightly compressing the bar while it is still hotto re-, 7

For exmove any unevenness in the bar and to partially consolidate thesame, continuously moving the bar through another heating zone to heatthe bar to a temperature substantially above the sintering temperatureof the metal bar removing the hot bar from the continuously movingsupport and continuously moving the hot bar between a pair of powerfulheated pressure rolls to compress the metal bar sufficiently to causenearly complete densification but insuffigfiently to cause anysubstantial elongation of the bar, continuously moving the hot barthrough another heating zone to efiect a further heating of the bar, andcontinuously moving the hot bar between a pair of powerful heatedpressure rolls to compress the bar sufliciently to cause completedensification and also substantial elongation of the bar.

3. The process for producing fabricated wrought copper products fromcopper powder which comprises heating the metal powder to a temperatureof about 600 C. to cause coherence of the metal powder particles to forma shaped coherent sintered porous bar, slightly compressing the barwhile it is still hot to remove any unevenness in the bar and partiallyconsolidate the same, continuing heating the bar to a temperaturebetween about 700 C. and 1050 (3., compressing the bar while it is stillhot between a pair of pressure rolls sufiiciently to cause nearlycomplete densification but insufficiently to cause any 8 substantialyelongation of the bar, again heating the bar to a temperature of about700 C. to about 1050 C., andfag'ain compressing the bar while it isstill hot between a pair of pressure rolls sufiiciently to causecomplete densification and also substantial elongation of the'bar.

References Cited in'the file of this patent UNITED .STATES PATENTS2,027,532 Hardy Jan. 14, 1936 2,134,366 Hardy Oct. 25, 1938 2,252,697Brassert Aug. 19, 1941 2,287,663 Brassert June 23, 1942 2,290,734Brassert July 21, 1942 2,300,048 Koehring Oct. 27, 1942 2,332,746 OltOct. 26, 1943 2,341,732 Marvin Feb. 15, 1944 2,350,179 Marvin May 30,1944 2,362,007 Hensel Nov. 7, 1944 2,362,701 Koehring Nov. 14, 19442,747,256 Wyatt et a1.v May 29, 1956 2,771,637 Silvasy Nov. 27, 1956OTHER REFERENCES Goetzel: Treatise on Powder Metallurgy, pages 440- 451,vol. 2, 1950.

1. THE PROCESS OF PRODUCING FABRICATED WROUGHT METAL PRODUCTS FROM METALPOWDER WHICH COMPRISES HEATING THE METAL POWDER TO FORM A SHAPEDCOHERENT SINTERED POROUS BAR, COMPRESSING THE BAR WHILE IT IS STILL HOTBETWEEN A PAIR OF PRESSURE ROLLS SUFFICIENTLY TO CAUSE NEARLY COMPLETEDENSIFICATION THEREOF BUT INSUFFICIENTLY TO CAUSE ANY SUBSTANTIALELONGATION THEREOF, AGAIN HEATING THE BAR TO A